Calculate percentage based on rolling sum of products vs the part replacements

All,

I am not sure if it is even possible, but I am trying to track a percentage of parts replacement based on rolling sum of three to four months, and how many parts were replaced during a certain month.

The chart below has serial number which shows up each time a part is replaced for that serial number.  I am trying to sum the number of serial numbers without including duplicates based on date installed.  Such that, in March I would like to i would like for the sum to be 17 products.  Then I would like to find the percentage of components changed in the month of March for that total of 17.

If that does not make it more confusing enough, I would like to for the report to only use the number of part replacements associated with the serial number in the rolling sum.  Such if an item was install in December and in come 8 months later a part was replaced, would not like to ensure that does not show up in the computation.

Not sure if thisis feasible or I communicated it correctly.